A geared turbofan engine is a type of turbofan airplane engine, similar to a turbojet. It consists of a geared ducted fan with a smaller diameter turbojet engine mounted behind it that powers the fan. Part of the airstream passes through the core of the engine, which includes low and high pressure compressors, a combustion chamber and high and low pressure turbines. The high and low pressure turbines drive the compressors and the fan.
The bypass ratio is the ratio of the amount of air entering the nacelle to the amount of air that passes through the core of the engine or the core nacelle. As the bypass ratio increases, the mean radius ratio of the fan and low pressure turbine increases. Consequently, if the fan is to rotate at its optimum blade speed, the low pressure turbine rotors will spin slowly, so additional low pressure turbine stages will be required to extract sufficient energy to drive the fan. Introducing a planetary reduction gearbox with a suitable gear ratio between the low pressure compressor shaft and the fan enables both the fan and low pressure turbine to operate at their optimum speeds.
Thus, in a geared turbofan, the fan produces most of the thrust and is driven through the planetary reduction gearbox, rather than being directly connected to the rest of the engine. The gearbox between the fan and low-pressure compressor and turbine allows the selection of the best possible operating speed for each engine section. Each runs much more efficiently, reducing the number of engine stages and parts.
Under normal operating conditions, the fan has a center of gravity that is coaxial with a central axis of the engine. The fan also has key natural vibratory frequencies which, by design, are higher than the maximum rotational frequency of the fan. By way of a non-limiting example, a turbofan engine having a cruising speed of 2,000-2,500 rpm and a full thrust/take-off speed of about 3,000 rpm may have key natural frequencies advantageously at least about 10% higher than the full thrust speed (e.g., about 3,300 rpm or 50.5 Hz).
During engine operation, a fan blade or a fragment thereof may become separated from the remainder of the fan (a so-called “fan blade-off” or “fan blade-out” event (FBO)) so that the center of gravity (center of mass) of the fan is displaced from the central axis. At least initially, bearings constrain the fan radially, so that it continues to rotate about the central axis rather than about an axis passing through the displaced center of gravity. However, the rotation of the displaced center of gravity about the central axis results in forces that may damage other engine components.
Upon a fan blade-off event, the engine ceases normal operation and produces no further power. However, it is typically not desirable to stop rotation of the engine's fan. If rotation of the fan were stopped, the engine would constitute an extreme source of aerodynamic drag. Such drag would be particularly significant in twin-engine aircraft wherein engines are mounted in wing nacelles. This is a common construction for many passenger aircraft. Thus, in twin-engine aircraft, the combination of drag from the stopped engine and thrust from the remaining engine would produce an excessive yawing moment not easily overcome by the aircraft rudder.
Accordingly, the damaged engine is advantageously allowed to rotate, driven by the air flow resulting from the forward velocity of the aircraft in a process called “windmilling”. A windmilling engine has significantly less aerodynamic drag than does a completely stopped engine. The potentially damaging imbalance forces are transmitted from the windmilling fan through the bearings to the support frame. To remain windmilling, the engine must resist damage to the turbine, bearings, etc. The engine must also be configured to avoid catastrophic damage, which may be caused by shaft failure, and which might permit engine parts to enter the high pressure turbine. If parts or debris enter the high pressure turbine, centrifugal forces may cause the parts or debris to puncture one or both of the nacelles, the fuselage or allow the engine to detach from the aircraft or damage the wing.